Sample exception message here:
System.IO.FileLoadException: Could not load file or assembly 'System.Text.Encoding.CodePages, Version=9.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. Could not find or load a specific file. (0x80131621)
The circumstance is that System.Text.Encoding.CodePages version 8.0 was already loaded, so the request for 9.0 failed. Ideally the message would say this.